[release testing] Heavier workaround for LVM-related race conditions

examples/release-testing

@@ -4,5 +4,6 @@
 for dist in $(awk '!/^#|^$/ {print $1}' /etc/xen-tools/distributions.conf | egrep -v '^(dapper|edgy)$'); do
     xen-create-image --dist $dist --force --verbose "$@" || break
     # Work around umount being sometimes too slow
-    sleep 2
+    sync
+    sleep 5
 done 2>&1 | tee xen-tools-release-testing.log